return M_BAD_ALIAS when trying to set non-existent canonical aliases

This is the error code named in the spec.

src/api/client_server/state.rs

@@ -232,7 +232,7 @@ async fn send_state_event_for_key_helper(
                     .is_none()
             {
                 return Err(Error::BadRequest(
-                    ErrorKind::forbidden(),
+                    ErrorKind::BadAlias,
                     "You are only allowed to send canonical_alias events when \
                      it's aliases already exists",
                 ));